How did you plan your route?With the help of travel book shop Stanfords, they put together a 9 sheet map, the equivalent of the A-Z with all of the pages stuck together. Wikipedia was really useful to see the geography of each individual Tube line. For example I could see that the Northern Line is in effect a figure eight with a line coming out of the bottom.

You weren't walking the whole 291 miles continually, so how long did the walk take?I started last July with the Victoria Line and finished with the Metropolitan Line just a few days before Christmas. I did a walk every other week. The Victoria Line took 8 hours in an afternoon. The sixty miles of the Piccadilly Line took two days. The first thirty-five miles of the Central Line took a day, and I was limping at the end of that. Which was a pity, as I had the other twenty-seven miles of it to do the next day. The worst was the Metropolitan Line which took 3 days to complete. The last of the 71 miles were completed in the snow.

Did you ever feel like giving up?No. I'm afraid that's good old-fashioned male obsessiveness for you – once I’d started I had to finish. Seriously, I agree with the quote that "It's better to travel than to arrive" and I found myself getting a strange kind of rush and enjoyment from the walks. Anything up to 30 miles makes you feel really good. Walking the Hammersmith and City Line and reaching Barking, the feeling of euphoria was amazing (who’d ever have thought that could happen in Barking?) . However, I found that anything over 30 miles starts to zap you. The end of the Piccadilly Line wasn't great.

What was the most surprising thing you learnt doing this?I was amazed at how much I was walking in and out of the countryside. On the Essex bit of the Central Line, for instance, there’s a whole chunk of farmland just off the A12 that you never get to see from the road. People outside of London think it’s full of arrogant and elitist people who don’t care about anywhere else. However the walk made me consider how much the capital is tied to the rest of the country. Most people I know who live in London are originally from somewhere else, and have strong family ties back in those places.

I also discovered lovely little things, like the story of the 18th Century beggar who drew on the mile long boundary wall of Kew Gardens at Kew Road. It was an artistic equivalent of busking – he drew outlines of every ship in the Navy Fleet.

Also many of my perceptions of places on the outskirts of London were challenged. Walking whole lines you go from glamorous to not very glamorous. I thought that Arnos Grove sounded like the worst place on the planet, but it was actually very nice.

I also discovered the origins of the Russian word for station - Вокзал. In 1840 the Russians sent a delegation to London to see how railways work. They were shown Vauxhall station and thought that Vauxhall or rather Vokzal (Вокзал) was the generic word for station.

You met the shouty man who was the voice of "Mind the Gap" on your travels. How did that happen?That was Tim Bentinck who played David Archer in The Archers. He'd been in touch with me before about one of my other books and as he lived on the Piccadilly Line, I thought I'd try to meet up. When he went to do the recording, he had no idea it was going to be for the Tube until he got to the studio. He was paid a flat fee of £200, despite the fact that his voice went on to be heard countless times.

People have problems pronouncing Holborn, and his friends always said that he didn't know how to pronounce it. But this was simply because he was asked to put the tiniest hint of an L in the middle.

Russell Square was one of the last remaining stations to use his voice and his wife works near there. She always said "Thank you darling", when she heard him say "Mind the Gap"

Finally, I often get asked this in interviews, what's your favourite Tube station?

Barons Court. It's very similar to the beautiful oxblood red stations designed by Leslie Green and Stanley Heap (such as Hampstead and Covent Garden). However it's actually slightly paler and different as it was designed by Harry Ford. Sadly, unless you live there, most people don't go out to see the outside and it's often overlooked.

I'd like to thank Mark for the interview and look forward to reading Walk the Lines when it comes out on July 14th.